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Course: Health and medicine > Unit 8
Lesson 3: Function of neurons and neurotransmittersOverview of neuron function
This video introduces the function and functional types of neurons. Neuron function involves processing and transmitting information. Key components include resting potential, action potentials, and the roles of dendrites, axons, and neurotransmitters. The nervous system contains different types of neurons, each with a unique role. By Matt Jensen. Created by Matthew Barry Jensen.
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- Wait...so action potentials happen faster with axons of a larger diameter? Is that it?(5 votes)
- Resistance within an axon is inversely proportional to the axon's diameter. Because the resistance is lower, the larger the axon's diameter, the faster the action potential can be transmitted.
A good way to think about it is to imagine water flowing through a pipe with a small diameter as compared to one with a large diameter. The water that is touching the side of the pipe experiences resistance (from friction) that slows it down. The water that isn't touching the sides of the pipe (so, the water in the middle) is not in contact with the pipe surface and so it does not experience that resistance from friction. Because it doesn't experience this resistance, it can travel at a faster speed.
In a large pipe, while the water on the edges experiences the resistance, there is a lot of water in the middle that can travel at this faster pace. Contrast that with a small pipe, which has less water in the middle, and so it has less water that can travel at a faster rate. The same is true in large vs. small diameter axons, except that it is the cell membrane that is the cause of the resistance that can impede the movement of a charge down the axon.(36 votes)
- Is the "trigger zone" being discussed atthe same as the axon hillock? If so, are those two words interchangeable? If "trigger zone" and "axon hillock" are not interchangeable, could you please explain the difference? Thanks! 1:48(13 votes)
- The trigger zone is an area near the axon hillock that causes voltage gated channels within the cell to open and initiate the propagation of the cell's action potential.
The axon hillock specifically refers to an area within the cell where the axon originates. It is cone shaped and is marked by a high density of intracellular microfilaments and a lack of Nissl bodies.(13 votes)
- Is there Interneurons in the Peripheral Nervous System? If yes, why wasn't it added in the video?(6 votes)
- no, interneurons are just present inside the CNS(3 votes)
- So, if action potentials are always the same size and duration, will neurotransmitter release always be the same in quantity/magnitude?(5 votes)
- Neurotransmitters are usually stored inside vesicles in terminal end of neurons. When action potential reaches terminal end, some of those vesicles are fused to cell membrane and transmitter of those vesicles are released to synaps gap. If there is a lot of action potentials without enough recovery time for neuron, terminal end can run out of transmitter-vesciles. That means that action potentials don't make terminal end release anymore neurotransmitters.
In normal case, if neurons have time for recovery, the amount of neutrotransmitters released are functionally constant. That means, there is no functional chance between action potentials. Of course the action which neurotransmitters cause depend on many other things too. For example one neuron takes input for many other neurons and then calculates should it or should it not release action potential - one neuron doesn't command other neuron by itself.
Also the action potentials of same neuron are not always same size and duration. Elctrical enviroment of a neuron makes an effect on neuron's function.(3 votes)
- Are one of the physical stimuli what we see? Say we are told to press a button as soon as we see a dot appear... Do neurons, in this case, carry our signal?(3 votes)
- Yes; neurons would carry the sensory information to the brain that allows you to recognize the dot, and then neurons would carry the motor stimuli to the relevant muscles which would make you press the button.(6 votes)
- "Input information usually comes in through the dendrites. Although less often, it'll come in through the soma or the axon." In what cases could the input skip dendrites and go straight into an axon or soma?(5 votes)
- I believe that this expression is somewhat problematic: " It is more negative inside the cell membrane whereas it is more positive outside the cell membrane". I believe that you should emphasize the fact that both outside and inside of the cell are positive(since both potassium and sodium are known as positive (+) ions). The difference is; inside the cell is LESS positive compared to outside of the cell.(3 votes)
- Actually you are partially correct. The inside of the cell does experience a net loss of positive charge. However, this leaves the cell to have excess of negatively charged proteins, hence, the negative charge.(4 votes)
- How does the action potential go with the neurotransmitter?(3 votes)
- I think you mean how do neurotransmitters spread the action potential, so I will try to explain this process. If you meant something different, please tell me so as a comment.
When an action potential reaches the end of an axon and hits the axon terminal, certain calcium gates in the axon terminal open, triggering an influx of calcium into the cell. The calcium causes little vesicles in the axon terminal (these vesicles contain neurotransmitters) to undergo exocytosis (meaning they leave the cell and the particles they contain [the neurotransmitters] are released out of the cell). These neurotransmitters cross a short gap, called the synaptic cleft, to reach another neuron. There are special receptors on these neurons that the neurotransmitters bind to. Once they had binded, they open certain ion channels or gates. If they open a sodium channel, for example, then the action potential will occur in the cell that the neurotransmitters are touching. Here is a picture that shows how a chemical synapse works (because what I have just described is called a chemical synapse):
https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjT59zjo7XNAhUEMyYKHUy0BvEQjRwIBw&url=http%3A%2F%2Fwww.zoology.ubc.ca%2F~gardner%2Fsynapses%2520-%2520presynaptic.htm&psig=AFQjCNECVytj5Zkk1tjvnORAX0sk3__Myw&ust=1466466551685320
There is also a khan academy video that can teach you how a synapse works in depth: https://www.khanacademy.org/science/health-and-medicine/human-anatomy-and-physiology/nervous-system-introduction/v/neuronal-synapses-chemical(3 votes)
- Are all the sensory neurons pseudo unipolar neurons?(3 votes)
- Sensory neurons are unipolar or bipolar neurons.(3 votes)
- What is the difference between afferent and sensory neurons?(3 votes)
- Afferent and sensory neurons are the same type of neuron.(3 votes)
Video transcript
In this video, I want to provide
an overview of neuron function, which I think of sort of
like how a gun functions. And we'll go into
a lot more detail on how a neuron functions
in later videos. But in this video. I just want to give a
bird's eye overview of it. The function of neurons
is to process and transmit information. Without input, most neurons
have a stable electrical charge difference across
their cell membrane, where it's more negative
inside the cell membrane and more positive outside
the cell membrane. And we call this the resting
membrane potential or just resting potential for short. And this resting
potential is really how the neuron is
going to be able to be excitable and respond to input. And I think of this
as similar to loading a gun by putting a bullet in it. Neurons receive excitatory
or inhibitory input from other cells or
from physical stimuli like odorant
molecules in the nose. Input information usually
comes in through the dendrites. Although less often, it'll
come in through the soma or the axon. The information from the
inputs is transmitted through dendrites or
the soma to the axon with membrane potential changes
called graded potentials. These graded
potentials are changes to the membrane potential away
from the resting potential, which are small in size
and brief in duration, and which travel
fairly short distances. The size and the duration
of a graded potential is proportional to the size
and the duration of the input. Summation, or an adding
together of all the excitatory and inhibitory graded
potentials at any moment in time occurs at the trigger zone,
the axon initial segment right here. This summation of
graded potentials is the way neurons process
information from their inputs. If the membrane potential
at the trigger zone crosses a value called
the threshold potential, information will then
be fired down the axon. So I like to think of
this process of summation of the excitatory and
inhibitory graded potentials at the trigger zone as analogous
to the trigger of a gun. In fact, that's why it's
called the trigger zone. I think of the graded
potentials as being like the finger on the
gun, that may be squeezing a little harder or relaxing. But once the trigger of
the gun is pulled back past a certain
threshold distance, a bullet will be fired
down the barrel of the gun, just like if the membrane
potential of the trigger zone crosses a threshold
value, information will be fired down the axon. The way information
is fired down the axon is with a
different kind of change to the membrane potential
called an action potential. An action potential is usually
large in size and brief in duration. But it's usually conducted
the entire length of the axon, no matter how long it
is, so that it can travel a very long distance,
just like a bullet usually has no trouble making it
down the barrel of the gun. And like a bullet traveling
through the barrel of a gun, action potentials tend to
travel very quickly down the length of the axon. Action potentials are different
than graded potentials because they're usually
the same size and duration for any particular
neuron, as opposed to the graded potentials,
whose size and duration depends on the size and
the duration of the inputs. Action potentials are
conducted faster along larger axons, axons
with a larger diameter, and along axons that have
a myelin sheath, that I've drawn in yellow here. When an action potential
reaches the axon terminals at the end of the
axon, information will then cross,
usually a small gap, to the target cell
of the neuron. And the way this happens
for most synapses where an axon terminal makes
contact with the target cell is by release of molecules
called neurotransmitters that bind to receptors
on the target cell and which may
change its behavior. Neurotransmitter is then
removed from the synapse. So it's reset to transmit
more information. And I think of this part as
similar to the bullet leaving the gun, to hit the target. The input information
that was converted into the size and the
duration of graded potentials is then converted into the
temporal pattern of firing of action potentials
down the axon. And this information
is then converted to the amount and the temporal
pattern of neurotransmitter release at the synapse. These steps are how neurons
transmit information, often over long distances. This is the general way that
neurons usually function. But there are multiple
functional types of neurons. So let's take a look
at some of those. Here I've drawn a few
different neurons, with their somas in red,
their axons in green, and their dendrites in blue. And I've drawn a
line here to separate between the central nervous
system on this side-- so I'll just write
CNS for short-- and the peripheral nervous
system on this side-- so I'll just write
PNS for short. And there's some
different ways we can categorize functional
types of neurons. The first way is the
direction of information flow between the CNS and the PNS. If a neuron like this
pseudounipolar neuron right here brings information
from the periphery in toward the central
nervous system, we call that an afferent neuron. Afferent, meaning it's
bringing information into the central nervous system. We can also call
this type of neuron a sensory neuron
because the information it's bringing into the
central nervous system involves information
about a stimulus. And a stimulus is
anything that can be sensed in the internal or
external environment, which is to say anything
inside the body or anything outside the body. These neurons are
carrying information away from the central nervous
system out into the periphery. So instead of calling
them afferent neurons, we call them efferent neurons. And there are two main
kinds of efferent neurons. The first we call motor neurons. Motor, which means movement. These are efferent neurons
that control skeletal muscle, the main type of
muscle that's attached to our skeleton,
that moves us around. These motor neurons are also
called somatomotor neurons or neurons of the
somatic nervous system. The other type of
the efferent neurons are called autonomic neurons. And these neurons
control smooth muscle, like the muscle around
our blood vessels; cardiac muscle, the muscle of
our heart; and gland cells, the cells of our glands
that secrete hormones into the bloodstream. These autonomic neurons are
also called visceromotor neurons or neurons of the
autonomic nervous system. Most neurons of the
central nervous system aren't any of these types
of neurons, however. They're like this
neuron, in that they connect other neurons together. So these are called
interneurons, neurons between neurons. And there are many interneurons
in the central nervous system, forming very complex pathways
for information to travel. So that while an
individual neuron is processing and
transmitting information, these complex
networks of neurons in the central
nervous system are doing even more
complex processing and transmitting of information.